1. Field of the Invention
This invention relates to an improved zinc acrylate particle composition and a method for the production thereof. Particularly, this invention relates to a zinc acrylate particle composition intended to overcome various problems which arises while zinc acrylate is incorporated in a rubber composition or a synthetic resin and kneaded therewith, and to a method for the production thereof.
This invention further relates to a golf ball which contains the improved zinc acrylate particle composition of this invention as a component thereof.
2. Description of the Related Art
Zinc acrylate is a compound useful as a cross-linking agent. It is incorporated, for example, in a rubber composition with the aim of improving the vulcanizability thereof or in a synthetic resin as a modifying agent.
As a method for obtaining zinc acrylate, a method which comprises causing acrylic acid to react with a zinc compound in an organic solvent, distilling the reaction solution to expel the organic solvent therefrom, and thereafter drying the residue of distillation (JP-B-58-14,416) and a method which comprises filtering off the organic solvent from the reaction solution and thereafter drying the residue of the filtration have been well known heretofore. These methods, however, suffer the reaction product to adhere seriously to an inner wall or a stirring vane of a reaction vessel or to agglomerate into agglomerates and, therefore, induce such problems as deteriorating the workability, degrading the yield, and inflicting damage to the relevant devices in consequence of the repetition of such works.
With an object of solving these problems, numerous methods which comprise coating the surface of zinc acrylate particles by adding such a higher fatty acid as stearic acid or a zinc salt thereof have been proposed in the case of using zinc acrylate as kneaded in a rubber composition (JP-A-52-154,436, U.S. Pat. No. 4,141,599, JP-A-60-94,434, and JP-A-02-218,649).
These methods, however, necessitate in addition to the step of inducing the reaction between acrylic acid and a zinc compound a step of adding to the produced zinc acrylate such a higher fatty acid as stearic acid or a zinc salt thereof thereby coating the surface of the zinc acrylate particles and, as a result, require a device proper for the step. Further, when the zinc acrylate thus formed is to be kneaded actually with a rubber composition, it is required to be in the form of such a fine powder as of not more than 43 μm (325 mesh) in particle size. The zinc acrylate which is formed by the method as mentioned above has a larger particle size and, therefore, must be pulverized into a fine powder. This pulverization into the fine powder, however, is at a disadvantage in not only entailing huge labor but also deteriorating the working atmosphere so much as to incite hygienic problems because the zinc acrylate powder during and after the pulverization process is unusually liable to scatter and form dust. Furthermore, the zinc acrylate in the form of fine powder is at a disadvantage in adhering fast to an inner wall and stirring vane of a kneading device or agglomerating into agglomerates while the powder is being kneaded with a rubber composition and consequently deteriorating the workability and, on account of the liability of the fine powder to a non-uniform dispersion, suffering the finished product to become heterogeneous and to degrade quality as well.
In consideration of these problems, a method for producing zinc acrylate which is substantially fine and readily pulverizable into fine powder and, even when kneaded with a rubber composition, capable of being uniformly dispersed and kneaded in a state very rarely generating fast adhesion or agglomeration has been proposed in the official gazette of U.S. Pat. No. 5,789,616. This method is claimed to produce improved zinc acrylate by causing acrylic acid and a higher fatty acid of 12-30 carbon atoms to react with zinc oxide in an organic solvent while continuing dispersion of the zinc oxide in the organic solvent in the presence of an anionic surfactant. The zinc acrylate to be produced in Examples 1-4 and Comparative Example 1 cited in the official gazette, however, invariably contain particles having particle size of not less than 500 μm at ratios exceeding 20% by mass of the whole particles. Thus, they are in their unmodified form at a disadvantage in failing to be smoothly kneaded in rubber without inducing cohering agglomerates and failing to be fully satisfactorily dispersed as well.
Such zinc acrylate as is smoothly kneaded in a rubber composition and possesses fully satisfactory dispersibility as well, therefore, has not been obtained to date.
Further, even when zinc acrylate is used as a component for a rubber composition of a golf ball as proposed in the official gazette of JP-A-11-9,720, it has fallen short of possessing fully satisfactory kneading property and dispersibility.